An early, simple, lightweight plugin API, audio effects only,
plugins have no editors/GUI of their own (Ardour provides one, however).

LV2

An extensible, full-featured plugin API, audio and MIDI, plugins can provide their
own GUIs but may use the
one Ardour provides instead.

AU

OS X only, full featured, audio and MIDI, plugins can provide their own GUI

VST

Plugins using Steinberg's VST plugin standard. Varies by platform:

on Linux

(native) Linux VST plugins fully supported (VST2.4)

on Windows

(native) Windows VST plugins fully supported (VST2.4)

on OS X

(native) macOS VST plugins fully supported (VST2.4) since Ardour 5.5

Windows VST Plugins on Linux

VST plugins for Windows, but being used on Linux. Not
supported by normal builds of Ardour. Read
more…

Adding/Removing/Copying Plugins

Within Ardour, plugins are just another type of Processor and so
the techniques for adding/removing/copying/moving processors apply to plugins
as well. These techniques are covered on the
Processor Box page.